1. Technical Field of the Invention
The present invention relates to the field of electronic circuitry, and more particularly, to output circuitry that drives an output signal off a chip.
2. Description of Related Art
Chips carrying electronic circuitry for use in all manner of devices, such as computers, appliances, automobiles, etc., can be manufactured by different processes. For example, a chip can be manufactured in a 5 volt process or in a 3 volt process. Generally speaking, the electronic devices (such as transistors) in a chip that is manufactured in a 5 volt process will have thicker layers than in a chip that is manufactured in a 3 volt process. It is also possible that different insulation materials can be used to differentiate between the processes. A 3 volt process suggests a higher integration density than a 5 volt process because all of the dimensions of the devices, including thickness, may be smaller.
The particular process by which a chip is manufactured will normally determine the manner in which the chip can be used. A chip manufactured by the 5 volt process will operate with logic voltage levels of 0 and 5 volts. Similarly, a chip manufactured by the 3 volt process will operate with logic voltage levels of 0 and 3 volts. For a device on the chip, such as a P channel transistor implemented in CMOS (complementary metal-oxide semiconductor) technology, this means that the transistor will be subjected to voltage differentials in the oxide layer between the gate and the substrate of either 5 or 3 volts, depending on whether the chip was manufactured by a 5 volt or a 3 volt process.
When the oxide layer of a device manufactured in a 5 volt process is subjected to 5 volts, (or a device manufactured in a 3 volt process is subjected to 3 volts) the device will normally operate reliably for the life expectancy of the chip. However, when the oxide layer of a device manufactured in a 3 volt process is subjected to a 5 volt differential, the oxide layer in the device will rapidly degrade. The device in the chip will then fail much earlier than the life expectancy of the chip. For example, a chip manufactured by the 3 volt process that has a normal life expectancy of 5 years, but has devices subjected to 5 volt differentials across the device, may fail in only 2 years. The reliability of such a 3 volt process chip placed in this situation will therefore be suspect.
There are advantages, however, provided by a chip that can produce 5 volt output signals but is manufactured in a 3 volt process. The current trend in chip manufacturing is to produce 3 volt process chips, due to the higher integration that is possible, the lower power consumption, and the reduced cost of the chips. However, there are still many chips and other apparatus that use logic voltage levels of 0 and 5 volts. It is therefore desirable to provide a 3 volt process chip that will be able to interface with chips that are manufactured in a 5 volt process. An obstacle to this interfacing (the production of 5 volt output signals) is the rapid degradation of the oxide layers of the devices in the 3 volt process chip when 5 volts are applied across the oxide layers. This rapid degradation causes the device to fail earlier than the expected life expectancy of the chip, making the chip unreliable.